Loose AirTag cover after battery replacement

The fit should be snug with no perceptible play, especially at the 3 and 9 o'clock positions. What you’re likely experiencing is either slight misalignment during reinstallation or wear on the cover lugs that grip into the AirTag's housing.

• Make sure the positive side (+) is facing up. Some batteries with a non-flat top (like Duracell’s button cells) can cause issues with the cover engaging properly.
• Remove the battery and the cover entirely. Then, carefully line up the three cover tabs with the grooves in the AirTag’s edge.
• Apply even downward pressure while twisting. You’ll feel a noticeable “click” or a stop when it’s fully locked in place. If you don't, try twisting back and forth slightly to seat the tabs.
• If the lugs on the cover (those small metal fingers underneath) look even slightly bent outward, they might not grip well anymore.
• In rare cases, battery thickness can vary slightly by brand or even batch. If you suspect the cover won’t lock down snugly despite proper alignment, try another brand (Sony and Panasonic are known to work well).

Finally, if you've tried all that and it still rocks or feels loose, it might be worth swapping the cover with one from a different AirTag (assuming that you have one available) temporarily to isolate if it’s just the cover that’s worn.

unless it's not screwed all the way around to fully seal it, then it sounds like that the casing have somehow been broken during the disasm when the replacement took place.